As a conventional speaker diaphragm, a diaphragm with paper used as a material is common. This is because paper is low in apparent density and has moderate rigidity and internal loss, so that the sound velocity of the diaphragm is relatively high (sound velocity=(E/p)1/2, E: elastic modulus, p: density). The higher the sound velocity, the follow-up capability of vibrations of the diaphragm in response to an electric signal is improved. Sound distortion is thereby reduced. However, in the case of applying paper to a speaker diaphragm, process steps such as papermaking are complicated, and the stability of quality degrades, raising problems in moisture resistance and water resistance.
Moreover, metallic materials such as titanium and aluminum are also used as a material of a diaphragm for the sake of rigidity greater than that of paper, however, they have a drawback of having small internal loss. This raises problems in frequency characteristics in that a sharp peak occurs in a high frequency range and distortion increases. Therefore, their uses are limited.
In order to improve workability, moisture resistance and water resistance, plastic materials such as a polypropylene resin are increasingly used as the material of a diaphragm, but they are disadvantageous in leading to insufficient sound velocity. Therefore, application of engineering plastics having great rigidity has been attempted.
For example, in Japanese Patent Laying-Open No. 6-225383 (Patent Literature 1), a material obtained by blending a cycloolefin polymer resin with a 4-methylpentene resin, and further adding mica and graphite thereto is applied to a diaphragm. For example, in Japanese Patent Laying-Open No. 2-276399 (Patent Literature 2), a diaphragm is molded from a material obtained by blending a liquid crystal polymer with a poly (4-methylpentene-1) resin, and compounding carbon fibers therewith.